Present invention relates to an electrophotosensitive material used in image forming apparatuses such as laser beam printer, electrostatic copying machine, plain paper facsimile, and combined machine with functions of them. The present invention also relates to a method of producing the electrophotosensitive material.
Recently, an organic photosensitive material comprising an electric charge generating material capable of generating electric charges by irradiation with light, an electric charge transferring material capable of transferring generated electric charges, and a binder resin has widely been used as the electrophotosensitive material. The organic photosensitive material is easily produced as compared with a conventional inorganic photosensitive material using an inorganic semiconductor material. Also the organic photosensitive material has advantages such as wide range of choice of materials such as electric charge generating material, electric charge transferring material and binder resin, and high functional design freedom.
The organic photosensitive material includes:
a photosensitive material comprising a single-layer type photosensitive layer wherein an electric charge generating material and an electric charge transferring material are dispersed in the same single layer made of a binder resin, and
a photosensitive material comprising a multi-layer type photosensitive layer wherein an electric charge generating layer containing an electric charge generating material and an electric charge transferring layer containing an electric charge transferring material are mutually laminated. The single-layer type photosensitive layer is formed by applying a coating solution, which is prepared by dispersing the respective components constituting the layer in a dispersion medium (organic solvent is exclusively used), on a conductive substrate, followed by drying-up. The multi-layer type photosensitive layer is formed by applying a coating solution for electric charge generating layer and a coating solution for electric charge transferring layer on a conductive substrate in this order or reverse order, followed by drying-up.
The electric charge generating material used in the photosensitive material includes various pigments according to the wavelength range at which the photosensitive material has the sensitivity. As the pigment for photosensitive material, which is sensitive to infrared to near infrared light such as semiconductor laser, infrared LED or the like, a phthalocyanine pigment is used. The phthalocyanine pigment includes metal-free phthalocyanine, titanyl phthalocyanine and the like, which differ in chemical structure.
The dispersibility of the pigment in the coating solution exerts a large influence on sensitivity characteristics of the photosensitive material. Therefore, various studies about a dispersion medium, which is capable of satisfactorily dispersing a pigment and a binder resin therein and also exhibits good stability after dispersion, have been made.
As the dispersion medium, a halogenated organic solvent has widely been used as a medium having excellent dispersibility of the pigment. However, use of the halogenated organic solvent tends to be avoided because of consideration to rise in environmental consciousness. Therefore, it has been required to secure the same dispersibility as that of the halogenated dispersion medium using a non-halogen dispersion medium.
As the non-halogen dispersion medium, for example, cyclohexanone, acetic esters, propanol, cellosolves and tetrahydrofuran are generally used. Also there is a report that alcohol is preferably used as the dispersion medium in case of xcex1-type titanyl phthalocyanine (Japanese Laid-open Patent Publication JP 08-160643 A).
Japanese Laid-open Patent Publication JP 03-033859 A suggests use of at least one selected from the group consisting of branched ester, branched alcohol and branched ketone as the dispersion medium capable of satisfactorily dispersing Y-type titanyl phthalocyanine, and polyvinyl butyral and/or silicone resin.
Japanese Laid-open Patent Publication JP 06-337525 A suggests use of an organic solvent having a hydroxyl group and an ether group in a molecule (so-called cellosolves) as the dispersion medium capable of satisfactorily dispersing a phthalocyanine pigment such as xcex1-type titanyl phthalocyanine, and polyvinyl butyral.
The present inventors have studied and found that a conventional coating solution using a phthalocyanine pigment in combination with a binder resin and the dispersion medium described in the respective prior arts described above has the following problems.
(i) It tends to cause precipitation or separation of a pigment immediately after preparation of the coating solution because the dispersibility of the pigment is still insufficient.
(ii) It is liable to cause precipitation or separation of a pigment during the storage of the coating solution for a long period of time because the dispersion stability of the pigment is insufficient, even if the dispersibility of the pigment is comparatively good.
(iii) Regarding an electrophotosensitive material comprising a layer containing a binder resin and a pigment (hereinafter, referred to as a xe2x80x9cpigment dipersion layerxe2x80x9d), which is formed by applying and drying the coating solution having the problem (i) or (ii), the sensitivity is drastically lowered. As used herein, the xe2x80x9cpigment dispersion layerxe2x80x9d includes a single-layer type photosensitive layer itself, and an electric charge generating layer among a multi-layer type photosensitive layer.
An object of the present invention is to provide a novel electrophotosensitive material which is free from lowering of the sensitivity and is superior in sensitivity characteristics.
Another object of the present invention is to provide a method of producing such an electrophotosensitive material.
The present inventors has studied intensively about the cause for lowering of the sensitivity of the photosensitive material as a result of formation of the pigment dispersion layer from the coating solution having the problem (i) or (ii) using a conventional dispersion medium, as described in (iii), and found the following fact.
The phthalocyanine pigment generally has strong agglomeration properties. Therefore, when the phthalocyanine pigment is used in combination with a conventional dispersion medium having poor dispersibility of the pigment, the pigment agglomerates to form agglomerates within a very short time until a pigment dispersion layer is formed by applying and drying the coating solution. Regarding the pigment dispersion layer thus formed, a large number of agglomerates of the pigment are formed in the layer. In such a pigment dispersion layer, the concentration of the pigment at the portion other than the agglomerates is drastically decreased as compared with the case where the pigment is uniformly dispersed in the layer. Therefore, the sensitivity at the portion is drastically lowered, thus lowering the sensitivity of the whole photosensitive material.
In some case, the degree of the lowering of the sensitivity at the portion other than the agglomerates is not large enough to impair the practicality because of less agglomeration of the pigment. In this case, however, there arises a new problem that image defects such as black spots and fogging are liable to be caused by lowering of charge at the agglomerate portion.
Therefore, the present inventors have studied about the cause for lowering of the dispersibility of the pigment and the dispersion stability of the coating solution in the coating solution using a conventional dispersion medium. As a result, they have found that the surface condition of particles of the pigment has a close relation with these characteristics.
In a dispersion medium, a binder resin is adsorbed on the surface of pigment particles and the pigment particles are stably dispersed in the dispersion medium by swelling of the binder resin with the dispersion medium. Therefore, when the amount of the resin adsorbed on the surface of the pigment particles becomes larger, the dispersibility of the pigment particles is improved and the dispersion stability of the coating solution is improved. On the other hand, when the amount of the resin adsorbed on the surface of the pigment particles becomes smaller, the dispersibility of the pigment particles is lowered and the dispersion stability of the coating solution is lowered.
It has hitherto been considered that a dispersion medium capable of satisfactorily dispersing xcex1-type thitanyl phthalocyanine therein is preferably conventional alcohol. However, when polyvinyl acetal such as polyvinyl butyral is used as the binder resin in the combination of the both, neither good dispersibility nor good dispersion stability could not be obtained. It is considered that this cause is based on the mechanism described above.
Comparing the conventional alcohol with polyvinyl acetal, the alcohol has strong polarity and strong hydrogen bonding properties as compared with polyvinyl acetal. Therefore, when polyvinyl acetal is dissolved in the alcohol and xcex1-type titanyl phthalocyanine is also dispersed, the alcohol is adsorbed on the surface of pigment particles (particles of xcex1-type titanyl phthalocyanine) in the amount larger than that of polyvinyl acetal. As a result, since the number of polyvinyl acetal adsorbed onto the surface of pigment particles is reduced, the dispersibility of the pigment particles is lowered and the dispersion stability of the coating solution is lowered.
Considering the mechanism described above, it is assumed that, when the polarity and hydrogen bonding properties of the alcohol are weakened, the number of the binder resin such as polyvinyl acetal absorbed onto the pigment particles increases, thereby to improve the dispersibility of the pigment particles and to improve the dispersion stability of the coating solution. Therefore, the present inventors have studied about the fact that strong polarity and hydrogen bonding properties due to an alcoholic hydroxyl group are inhibited by introducing an electron attractive group such as xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94,  greater than Nxe2x80x94,  greater than Cxe2x95x90O or the like into a molecule of the alcohol. As a result, they have found that, when using alcohol having a  greater than Cxe2x95x90O group in a molecule of the formula (1): 
wherein R1, R2 and R3 are the same or different and represent a hydrogen atom or an alkyl group, which pertains to alcohol in a broad sense but is not specifically disclosed in the prior art, and which has never been used in the electrophotosensitive material, the dispersibility of the pigment in the coating solution and the dispersion stability of the coating solution are improved, thereby making it possible to form a pigment dispersion layer having excellent dispersibility of the pigment.
Accordingly, the invention of claim 1 is directed to an electrophotosensitive material comprising a photosensitive layer having at least a layer containing a binder resin and a pigment (pigment dispersion layer), wherein the layer is formed by dispersing the binder resin and the pigment in a dispersion medium comprising at least alcohol represented by the formula (1): 
wherein R1, R2 and R3 are the same or different and represent a hydrogen atom or an alkyl group, to prepare a coating solution, and applying and drying the coating solution.
The constitution of the electrophotosensitive material of claim 1 is preferably employed in case the pigment includes a phthalocyanine pigment having strong agglomeration properties.
Accordingly, the invention of claim 2 is directed to the electrophotosensitive material according to claim 1, wherein the pigment includes at least a phthalocyanine pigment.
The constitution of the electrophotosensitive material of claim 1 is also preferably employed in case the binder resin includes polyvinyl acetal which has weak polarity and hydrogen bonding properties as compared with conventional alcohol.
Accordingly, the invention of claim 3 is directed to the electrophotosensitive material according to claim 1, wherein the binder resin includes at least polyvinyl acetal.
Preferable alcohol of the formula (1) is diacetone alcohol represented by the formula (1xe2x80x941). 
Accordingly, the invention of claim 4 is directed to the electrophotosensitive material according to claim 1, wherein the alcohol of the formula (1) is diacetone alcohol.
The invention of claim 5 is directed to the electrophotosensitive material according to claim 1, wherein the existence of agglomerates of the pigment is not found in a 0.25 mm square region in actual size of the pigment dispersion layer at any of plural positions.
In case the existence of at least one agglomerate of the pigment is found, the sensitivity is likely to be lowered drastically by the formation of the agglomerate of the pigment. On the other hand, the electrophotosensitive material, wherein the existence of any agglomerate of the pigment is not found in the pigment dispersion layer, is superior in sensitivity characteristics and does not cause defects such as black dots and fogging on the image.
The present inventors also have studied about the procedure for preparation of the coating solution so as to improve the dispersibility of the pigment and to improve the dispersion stability of the coating solution, using various dispersion mediums including the alcohol of the formula (1).
As a result, they have found that, a coating solution is prepared by dispersing a binder resin and a pigment in an arbitrary organic solvent, drying the dispersion to remove the organic solvent to obtain a powder in which the binder resin is adsorbed on the surface of pigment particles, and dispersing the powder in a dispersion medium, the dispersibility of the pigment and the dispersion stability of the coating solution are improved, thereby making it possible to form a pigment dispersion layer having excellent dispersibility of the pigment in the same manner as described above.
That is, the invention of claim 6 is directed to an electrophotosensitive material comprising a photosensitive layer having at least a layer containing a binder resin and a pigment (pigment dispersion layer), wherein the layer is formed by dispersing the binder resin and the pigment in an organic solvent, drying the dispersion to remove the organic solvent to prepare a powder in which the binder resin is adsorbed on the surface of pigment particles, and dispersing the powder in a dispersion medium to prepare a coating solution, and applying and drying the coating solution.
The constitution of the electrophotosensitive material of claim 6 is preferably employed in case the pigment includes a phthalocyanine pigment having strong agglomeration properties.
Accordingly, the invention of claim 7 is directed to the electrophotosensitive material according to claim 6, wherein the pigment includes at least a phthalocyanine pigment.
The constitution of the electrophotosensitive material of claim 6 is also preferably employed in case the binder resin includes polyvinyl acetal which has weak polarity and hydrogen bonding properties as compared with conventional alcohol.
And when polyvinyl acetal is used, preferable dispersion medium includes at least one selected from the group consisting of alcohol represented by the formula (1): 
wherein R1, R2 and R3 are the same or different and represent a hydrogen atom or an alkyl group, an alicylic ketone represented by the formula (2): 
wherein R4, R5, R6, R7, R8, R9, R10, R11, R12 and R13 are the same or different and represent a hydrogen atom or an alkyl group, alcohol represented by the formula (3):
CnH2n+1OHxe2x80x83xe2x80x83(3) 
wherein n represents an integer of not more than 4, and a cellosolve represented by the formula (4):
CmH2m+1OC2H4OHxe2x80x83xe2x80x83(4) 
wherein m represents an integer of not more than 2.
Accordingly, the invention of claim 8 is directed to the electrophotosensitive material according to claim 6, wherein the binder resin includes at least polyvinyl acetal and the dispersion medium includes at least one selected from the group consisting of alcohol represented by the formula (1), an alicylic ketone represented by the formula (2), alcohol represented by the formula (3), and a cellosolve represented by the formula (4), as above mentioned.
The invention of claim 9 is directed to the electrophotosensitive material according to claim 6, wherein the existence of agglomerates of the pigment is not found in a 0.25 mm square region in actual size of the pigment dispersion layer at any of plural positions. In case the existence of at least one agglomerate of the pigment is found, the sensitivity is likely to be lowered drastically by the formation of the agglomerate of the pigment. On the other hand, the electrophotosensitive material, wherein the existence of any agglomerate of the pigment is not found in the pigment dispersion layer, is superior in sensitivity characteristics and does not cause defects such as black dots and fogging on the image.
The present inventors also have found that, in an electrophotosensitive material wherein a pigment dispersion layer is formed by using diacetone alcohol among the alcohol represented by the formula (1), a small amount of diacetone alcohol must be remained in the formed pigment dispersion layer.
Diacetone alcohol is used alone or in combination with the other dispersion medium because the dispersibility of the pigment is improved by increasing the number of the binder resin to be adsorbed onto the surface of the pigment, as described previously. Accordingly, in order to form the pigment dispersion layer while maintaining good pigment dispersibility, diacetone alcohol, which swells the binder resin, is preferably remained until the completion of the formation of the pigment dispersion layer.
To the contrary, regarding the pigment dispersion layer wherein diacetone alcohol is not remained after formation, diacetone alcohol has already evaporated at any stage of the formation process and the dispersibility of the pigment is not secured thereafter and, therefore, the agglomerate is frequently formed. On the other hand, the pigment dispersion layer wherein diacetone alcohol is remained after the formation is a layer which is free from agglomerates and is superior in dispersibility of the pigment.
Therefore, the present inventors have further studied about the method of detecting whether or not diacetone alcohol is remained in the pigment dispersion layer after formation, thus completing the present invention.
The invention of claim 10 is directed to an electrophotosensitive material comprising a photosensitive layer having at least a layer containing a binder resin and a pigment (pigment dispersion layer), wherein the layer contains diacetone alcohol, characterized in that a sample collected from the layer has peaks at a detection time ranging from 5.3 to 7.5 minutes in gas chromatography at a thermal decomposition temperature higher than a boiling point of diacetone alcohol, and that the sample has peaks at the positions where a m/z value as a ratio of a mass to an electric charge is 43, 59 and 101, in a mass spectrum detected by a specific ion detection process of at least one peak of the gas chromatographic peaks.
The constitution of the electrophotosensitive material of claim 10 is preferably employed in case the binder resin includes polyvinyl acetal and the pigment includes a phthalocyanine pigment.
Accordingly, the invention of claim 11 is directed to the electrophotosensitive material according to claim 10, wherein the binder resin includes at least polyvinyl acetal and the pigment includes at least a phthalocyanine pigment.
The invention of claim 12 is directed to a method of producing an electrophotosensitive material comprising a photosensitive layer having at least a layer containing a binder resin and a pigment (pigment dispersion layer), which comprises steps of:
dispersing the binder resin and the pigment in a dispersion medium comprising at least alcohol represented by the formula (1): 
wherein R1, R2 and R3 are the same or different and represent a hydrogen atom or an alkyl group, to prepare a coating solution; and
applying and drying the coating solution to form a layer containing the binder resin and the pigment.
According to the method of claim 12, the electrophotosensitive material of claim 1 (hereinafter referred to as a xe2x80x9cfirst electrophotosensive materialxe2x80x9d) having excellent characteristics described above can be produced efficiently.
The invention of claim 13 is directed to a method of producing an electrophotosensitive material comprising a photosensitive layer having at least a layer containing a binder resin and a pigment (pigment dispersion layer), which comprises the steps of:
dispersing the binder resin and the pigment in an organic solvent and drying the dispersion to remove the organic solvent to prepare a powder in which the binder resin is adsorbed on the surface of pigment particles;
dispersing the powder in a dispersion medium to prepare a coating solution; and
applying and drying the coating solution to form a layer containing the binder resin and the pigment.
According to the method of claim 13, the electrophotosensitive material of claim 6 (hereinafter referred to as a xe2x80x9csecond electrophotosensitive materialxe2x80x9d) having excellent characteristics described above can be produced efficiently.